If stars moving away/towards Earth appear blue/red shifted, how do we know their colours are red/blue because they are moving, and not just that they are a red/blue coloured star that’s relatively stationary?

Question

I copied the wording from a Quora post because all the answers were ELIhave a PHD in astrophysics.

In: 6

Answer 1

Spectral Lines

Stars may appear “White” to our eyes but in reality they give off light in all sorts of different “colors” that mixed together only appear white to our eyes.

When we say that the light was red-shifted it doesn’t mean that the star itself appears red, just that all of its light was shifted towards the red end of the spectrum.

“Red” in this case means lower frequency and “blue” higher frequency. Red-shifted means all the light the star gives of was shifted a bit towards the lower frequency end of the spectrum. In practice that means that some light that was to high frequency to see otherwise will move into the part of the spectrum people can see and some part at the lower end move out of it.

Red is not really a description of the end result as much as a direction describing the change.

If a star gives of light in all the colors and the light gets red shifted or blue shifted it will not look different. However it isn’t quite “all the colors” in the real world.

Atoms and molecules give of and absorb light in very specific frequencies.

Think of it as a rainbow with some dark bands interrupting the usual continuous spectrum of colors.

Those lines are linke fingerprints for elements. We can tell what elements there are in an object by the lines in its spectrum.

If we see light from a distant star that looks exactly like we expect from such a star only everything is moved a bit towards the red or blue side, we can guess that the star was red-shifted or blue shifted.

We see a distinctive and recognizable pattern of lines in the spectrum shifted a bit towards the red or the blue side of the spectrum.